Theoretical and Experimental Evaluation of Heading Instability of Moored Ships in Waves and Winds

Floating oil and LNG facilities consist of ship-type floating production, storage and offloading (FPSO) hull that receives fluids from a subsea reservoir, which is processed and stored before offloading to a shuttle tanker. The new facilities technology will allow suppliers of natural gas to effectively monetize both onshore and offshore gas reserves. FPSOs operate in various offshore environments and therefore require survival in complex seas made up of non-collinear winds, wind-driven seas and large ocean swells. Such bi-directional waves along with wind conditions appear in regions like West of Africa and Brazil (Campos Basin), where the 100-year condition comprises of extreme swells in addition to local waves of varying degrees of severity. A nascent interest of the offshore industry has been on studying the global performance of FPSO and Floating LNG systems in complex sea conditions in deep waters. Many of these vessels are held in station by a mooring system that is connected to the platform by a rotating turret. The heading stability of the vessel and its ability to self-align with respect to oncoming environmental forces is studied in this study. The heading instability in regular waves including long swells and wind is evaluated. The effect of viscous damping on the heading stability of FPSOs is analyzed. Time domain simulations conducted using industry-standard software. Numerical results shows that additional damping in heave and pitch minimizes the heading stability in regular waves especially for wavelengths above λ/L=1.5. The simulation and measured responses of the FPSO to different waves and steady wind speeds show that the effect of the wind on the heading instability is considerable for higher wind speeds. Experiments at a 1:120 scale conducted in a wave basin facility for two storm conditions offshore Brazil and West of Africa show that the heading angle is self-limiting to a range of ±20deg with respect to head sea direction. Simulating the wind sea and swell with suitable spectrum models approaching from different directions show almost double the heading angle range seen in experiments. If similar numerical analyses were part of a design process, it is conceivable that this could result in incorrect predictions of the weathervaning of the platform, and hence the latter’s global motions in response to design storm conditions. It is shown here that in a bi-directional seastate, the interaction between the two wave components can seriously alter the drift loads on the platform. Following the pioneering the work of Dr. Jo Pinkster, a cross-wave correction is added to the time domain simulations to account for the interaction. Implementing this correction brings the numerical results more in line with the experimental findings. The cross-wave correction term is quantified and recommendations are made for better simulations and experiments of such platforms